The modulation of transcription factor activity by protein-protein interaction has emerged as a central theme in transcriptional regulation. The mammalian transcription factor E2F was first described as a nuclear activity that binds to the adenovirus (Ad) E2a promoter. The binding of E2F to an inverted repeat in the E2a promoter is induced by the Ad E4-6/7 protein and correlates with transcriptional activation. Functional E2F recognition sites are present in the promoter regions of cellular genes involved in the regulation of cell growth. The association of E2F with cellular proteins that inhibit cell proliferation (members of the retinoblastoma. Rb. product family) varies with specific stages of the cell cycle. Inhibition of cell proliferation, a hallmark of normal Rb function, is relieved when the Rb-E2F interaction is lost by mutation of Rb or dissociated by the DNA tumor virus transforming proteins T-antigen. E1A or E7. This implicates E2F as a target in Rb signaling pathways and transcriptional regulation linked to E2F as one of the molecular mechanisms by which Rb functions. E2F activity represents the heterodimeric interaction of members of two protein families: the E2F proteins and the DP proteins. Specific members of these protein families have been shown to have transforming properties, and to induce S phase entry and apoptosis when overexpressed in cells. Taken together, these results suggest that E2F activity plays a central role in the regulation of cell growth. The regulation of E2F activity by protein interactions will not be explained without a definition of E2F activity in terms of its protein components and the function of its regulatory binding partners. The specific aims are: 1. To compare and contrast the function and regulation of E2F-1 and E2F-4 as paradigms of E2F family members that are regulated by Rb and p107/p130, respectively; 2. To study the role of the Ad E4-6/7 protein, and associated cdk2, in the regulation of E2F-directed transcription; 3. To analyze the structure and function of cellular analogues to the Ad E4-6/7 protein.